Computed tomography (“CT” or “CAT”) scanning systems are generally known in the art. The first CT scanners used a source of X-rays directed as a beam and a single detector to detect the amount of X-rays passing through the scanned object. During a scan of an object, the source and detector are passed through a line on the object, then the source and detector are moved relative to the object and scanned through another line on the object.
So called “second-generation” CT scanners use a fan-shaped X-ray beam and a corresponding plurality of detectors arranged along the fan. Similar to earlier CT scanners, a second-generation CT scanner may be moved relative to the scanned object to collect a full set of readings on the object. In between scans, the object may be rotated to expose a different portion of the object to the X-ray source. Usually, this rotation amount is close to or equal to the total angle spanned by the X-ray fan. In other prior scanners, the object may be rotated during translation or movement across the scanner.
So called “third-generation” CT scanners also use a fan-shaped (or cone-shaped) X-ray beam and a corresponding plurality of detectors arranged along the fan. In contrast to second-generation CT scanners, a third-generation CT scanner collects data many times during a full rotation of the scanned object, typically without translation or movement of the object across the scanner.
For any typical CT scanner, data is collected from each scan or view of the scanned object into an array that is manipulated by a computer to provide a variety of images of the object. These provided images are called reconstructions or reconstructed images. Several known algorithms exist for creating the reconstructed images of an object scanned by CT scanners. These algorithms use various geometric values relating to the physical CT scanning system to manipulate the collected scan data into the reconstructed images.
A common goal for imaging systems, such as CT scanning systems, is to provide images with the highest resolution possible. The resolution of a given system is limited by the effective beam width, which is dependant on the geometry of the system, and on the X-ray sampling. When the beam width is large, the image appears smeared and high-frequency detail is reduced. When the sample spacing is large, the high-frequency detail is lost altogether. Nyquist's criterion, applied to CT, says that to prevent losing any information due to sampling, the sample spacing should be less than half of the reciprocal of the highest frequency left in the data after it has been blurred by the beam width. To a first-degree approximation, this means that for good sampling, the sample spacing should be at most half the effective beam width, though to a lesser extent performance can be further improved by having even finer sampling. Conventional third-generation scanners inherently violate Nyquist's criterion and therefore have especially poor X-ray sampling. The reason for this is because for third generation, the sample spacing is the same as the detector pitch, and the detector pitch is inherently at least as large as the effective beam width.
One known method to get good X-ray sampling is to use second-generation scanning, where one can get good sampling by choosing the translation increment to be smaller than the effective beam width by an arbitrary amount. Second-generation scanning, however, is fairly inefficient because during much of the scan time the scan object is in positions where a significant portion of the X-ray beam does not intersect the scan object.
There are also several known methods for improving the sampling of a third-generation scan. One known method is to position the rotation-axis in such a place that the projection of the rotation axis onto the detector is exactly one quarter of the way between two neighboring detector elements. This method, however, requires that when changing slice positions, the rotation axis does not move from side to side. For medical geometries, where the source and detector are on a rotating assembly and the patient is fed through the middle, it is feasible to keep the rotation axis at a fixed location relative to the source and detector. For industrial geometries where the table rotates relative to a fixed source and detector, however, the table may shift from side-to-side as it is raised or lowered. Thus, the position of the rotation axis within the scanning plane may change with table height, and it is rather difficult to maintain the quarter-detector constraint on the rotation axis for all table elevation-positions. Furthermore, even if the position of the rotation axis can be forced to remain constant, this method can at best double the sampling of conventional CT—it typically cannot be extended to give further improvement.
Another known method to improve the sampling of a third generation scan is to use multiple X-ray sources, either by switching between multiple independent sources, or by moving the focal spot of a single source between multiple positions by means of electromagnetic deflection. X-ray sources with multiple focal spots, however, can be prohibitively expensive, and care must be taken to synchronize the source-switching with the detector sampling times.
Another known method to improve the sampling of a third generation scan is to take multiple scans, each with the detector in a slightly shifted position relative to the scanned object, then interleave the data from the scans. Many scanners, however, are not built with a detector motion axis, and for interleaving to produce mathematically correct data, the detector must be kept focused on the source.
Another known method to increase the X-ray sampling in a third-generation CT scanning system is to interleave data from two or more scans where the object has been rotated and/or translated effectively a fraction of a beam width. Interleaving, however, fails to provide mathematically correct data arrays in the case of translating the object with a flat detector array, with or without rotation, or, in the case of translating the object with a curved detector array, without rotation. For these cases when interleaving does not produce mathematically correct images, only a small number of scans may be interleaved before the maximum benefit of doing so is realized. The case where interleaving is indeed mathematically correct is where the object is shifted while simultaneously rotating it a fraction of an angular detector pitch. For systems with heavy objects, dense detector arrays, and/or a small fan angle, mechanical limitations can make the required small precise rotation rather difficult to achieve physically.
Another goal of CT scanning systems is to perform scans at the highest possible speed to improve the throughput of the scanner when scanning objects or to decrease the time for the scanning of a patient. Often, speed and resolution are incompatible goals where speed must be sacrificed for higher resolution and vice versa. Typically, third-generation scanning is used when high speed is the top priority, whereas second-generation scanning is often used when high resolution is the top priority. Ideally, the benefits of both scanning methods should be realized.
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